WO2016046868A1 - Positive active material for lithium ion secondary battery, positive electrode material, and lithium ion secondary battery - Google Patents

Abstract

The present invention addresses the problem of providing a lithium ion secondary battery which has a high capacity in both low-rate and high-rate discharges and is reduced in open circuit voltage (OCV) hysteresis arising between the charge and the discharge. The problem can be solved with a positive electrode material comprising: a first positive active material which is a positive active material represented by the empirical formula Li_xNi_aMn_bM_cO₂ (wherein 1.09<x<1.20, 0.21<a<0.42, 0.42<b<0.63, 0≤c≤0.02, x+a+b+c=2.0, and M is at least one element selected from among V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc.) and which has a specific surface area of 2.5 m²/g or less; and a second positive active material which has a similar composition and has a specific surface area of 3 m²/g or greater.

Description

Positive electrode active material for lithium ion secondary battery, positive electrode material, and lithium ion secondary battery

The present invention relates to a positive electrode active material for a lithium ion secondary battery, a positive electrode material, and a lithium ion secondary battery including the same.

The problem with electric vehicles is that the energy density of the drive battery is low and the distance traveled by one charge is short. Therefore, there is a need for a secondary battery that is inexpensive and has a high energy density.

Lithium ion secondary batteries have a higher energy density per weight than secondary batteries such as nickel metal hydride batteries and lead batteries. Therefore, application to electric vehicles and power storage systems is expected. However, in order to meet the demands of electric vehicles, it is necessary to further increase the energy density. In order to realize high energy density, it is necessary to increase the energy density of the positive electrode and the negative electrode.

As a high energy density positive electrode active material, a layered solid solution compound represented by Li ₂ MO ₃ —LiM′O ₂ is expected. The layered solid solution compound is a solution that dissolves electrochemically inactive Li ₂ MO ₃ and electrochemically active LiM′O ₂ in a solid solution and draws out a high capacity while utilizing high activity properties. . The layered solid solution compound can also be represented by the composition formula Li _{1 + x} M _1-x O ₂ .

Patent Document 1 discloses a solid solution formed by combining a plurality of solid solution compounds having different median diameters in a solid solution positive electrode material using a solid solution compound of Li [Li _1/3 M ¹ _2/3 ] O ₂ and LiM ² O _2. It has been reported that the tap density is improved by the positive electrode material (M ¹ is one or more metal elements and the average valence is tetravalent, and M ² is one or more metal elements and is an average. The valence is trivalent.)

JP 2012-059528 A

The positive electrode material disclosed in Patent Document 1 has a problem that although a high energy density is obtained at a low rate, the energy density at a high rate is poor. Furthermore, this material has hysteresis in the open circuit voltage (OCV: Open circuit voltage) during the charging process and discharging process. That is, the OCV differs depending on the charging process and the discharging process, and there are two OCVs in the same SOC. For this reason, there is a problem that it is difficult to detect the state of charge (SOC) of the battery from the voltage. If the state of charge cannot be accurately grasped, it is necessary to allow a surplus in the remaining capacity of the battery, and the capacity of the battery that can be used decreases.

Therefore, an object of the present invention is to provide a lithium ion secondary battery having a high energy density at a high rate and a small OCV hysteresis in the charging process and the discharging process.

Positive electrode material for a lithium ion secondary battery according to the present invention, the composition formula _{Li x Ni a Mn b M c} O 2 (1.09 <x <1.20,0.21 <a <0.42,0.42 <B <0.63, 0 ≦ c ≦ 0.02, x + a + b + c = 2.0, M is at least one element of V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc.) And a positive electrode active material having a specific surface area of 2.5 m ² / g or less.

Further, another positive electrode material for a lithium ion secondary battery according to the present invention uses the positive electrode active material as a first positive electrode active material, has a similar composition range, and has a specific surface area of 3 m ² / g or more. It contains the 2nd positive electrode active material, It is characterized by the above-mentioned.

According to the present invention, it is possible to provide a lithium ion secondary battery having a high energy density even in a high rate discharge and having a small OCV hysteresis.

It is sectional drawing which shows the structure of a lithium ion secondary battery typically.

<Positive electrode material>
When a lithium ion secondary battery is employed in an electric vehicle, it is expected that a high energy density can be obtained even in a large current discharge, and that SOC can be detected with high accuracy from the battery voltage. In order to increase the travel distance per charge, the battery is required to have a high energy density. A lithium ion secondary battery using a layered solid solution compound as a positive electrode active material can be expected to have a high energy density at a low rate, but the energy density at a high rate is low, and it is difficult to detect the SOC of the battery from the battery voltage. There are issues such as.

In the lithium ion battery, the SOC is detected from the battery voltage. The hysteresis in the OCV in the charging process and the OCV in the discharging process means that the OCV in the charging process is different from the OCV in the discharging process in the same SOC. That is, there are two SOCs corresponding to the same potential. When the difference between the two SOCs at the same potential is large, a large error occurs when the SOC is detected from the OCV, so that it is difficult to detect the SOC from the battery voltage. Therefore, in order to detect the SOC with high accuracy, it is necessary to suppress the hysteresis of the OCV.

Results inventors studied intensively, composition formula in the layered solid solution compound _{Li x Ni a Mn b M c} O 2 (1.09 <x <1.20,0.21 <a <0.42,0.42 < b <0.63, 0 ≦ c ≦ 0.02, x + a + b + c = 2.0, M is at least one element of V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc.) The positive electrode active material (first positive electrode active material) having a specific surface area of 2.5 m ² / g or less has a high energy density at a high rate, and can reduce OCV hysteresis in the charging and discharging processes. . Moreover, the energy density at a high rate can be further improved by mixing a positive electrode active material (second positive electrode active material) having a similar composition range and a specific surface area of 3 m ² / g or more. I found out.

By using the positive electrode material having this feature, it is possible to provide a positive electrode material having a high energy density even in a high rate discharge and having a small OCV hysteresis.

In the first positive electrode active material, x represents the proportion of Li in Li _x Ni _a Mn _b M _c O ₂ . When x is 1.09 or less, the amount of Li contributing to the reaction is reduced, and a high capacity cannot be obtained. On the other hand, if x is 1.2 or more, the crystal lattice becomes unstable, and the discharge capacity decreases.

A represents the proportion of Ni. When a is 0.21 or less, Ni mainly involved in the charge / discharge reaction decreases, and the capacity decreases. On the other hand, if a is 0.42 or more, the oxygen activation reaction in the initial charging process is unlikely to occur, and thus the capacity decreases.

B represents the ratio of Mn. When b is 0.42 or less, the activation reaction of oxygen hardly occurs, and thus the capacity decreases. On the other hand, if b is 0.63 or more, the amount of Ni that contributes to the reaction is reduced, so the capacity is reduced.

C represents the ratio of M (M is at least one element such as V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu). When c is larger than 0.02, the ratio of the elements contributing to the reaction is lowered, so that the discharge capacity is lowered.

When the specific surface area is 2.5 m ² / g or less with the above composition, the filling rate of the positive electrode active material is improved, so that the density can be increased and a high volume energy density can be achieved.

On the other hand, in a compound in which Ni is divalent and Mn is tetravalent as represented by the layered solid solution compound represented by the composition formula Li _1.2 Ni _0.2 Mn _0.6 O ₂ , the electrode density increases when the specific surface area is reduced. However, since the capacity at a high rate rapidly decreases, it is impossible to achieve both a high electrode density and a high capacity at a high rate. In order to achieve both characteristics, it is necessary to have the composition shown above.

Furthermore, the energy density per unit volume can be improved by mixing the present material with a second positive electrode active material having the same composition and a specific surface area of 3 m ² / g or more. The second positive electrode active material may have the same composition as the first positive electrode active material or a different composition as long as the composition range is satisfied. The ratio of the second positive electrode active material to the total amount with the first positive electrode active material is preferably 40% or less by mass ratio.

<Lithium ion secondary battery>
A lithium ion secondary battery according to the present invention includes the above positive electrode material. By using the above positive electrode material for the positive electrode, a high energy density can be obtained and the OCV hysteresis is small in a high rate discharge. The lithium ion secondary battery according to the present invention can be preferably used for an electric vehicle.

A lithium ion secondary battery includes a positive electrode including a positive electrode material, a negative electrode including a negative electrode material, a separator, an electrolytic solution, an electrolyte, and the like.

The negative electrode material is not particularly limited as long as it is a substance that can occlude and release lithium ions. Substances generally used in lithium ion secondary batteries can be used as the negative electrode material. For example, graphite, a lithium alloy, etc. can be illustrated.

As the separator, those generally used in lithium ion secondary batteries can be used. Examples thereof include polyolefin microporous films and nonwoven fabrics such as polypropylene, polyethylene, and a copolymer of propylene and ethylene.

As the electrolytic solution and the electrolyte, those generally used in lithium ion secondary batteries can be used. For example, diethyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, methyl acetate, ethyl methyl carbonate, methyl propyl carbonate, dimethoxyethane and the like can be exemplified as the electrolytic solution. Further, as the electrolyte, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN Examples thereof include (CF ₃ SO ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₃ .

An embodiment of the structure of a lithium ion secondary battery according to the present invention will be described with reference to FIG. The lithium ion secondary battery 10 includes an electrode group having a positive electrode 1 in which a positive electrode material is applied on both sides of a current collector, a negative electrode 2 in which a negative electrode material is applied on both sides of the current collector, and a separator 3. The positive electrode 1 and the negative electrode 2 are wound through a separator 3 to form a wound electrode group. This wound body is inserted into the battery can 4.

The negative electrode 2 is electrically connected to the battery can 4 via the negative electrode lead piece 6. A sealing lid 7 is attached to the battery can 4 via a packing 5. The positive electrode 1 is electrically connected to the sealing lid 7 via the positive electrode lead piece 8. The wound body is insulated by the insulating plate 9.

The electrode group may not be the wound body shown in FIG. 1, but may be a laminated body in which the positive electrode 1 and the negative electrode 2 are laminated via the separator 3.

Hereinafter, the present invention will be described in more detail using examples and comparative examples, but the technical scope of the present invention is not limited thereto.

<Preparation of first positive electrode active material (specific surface area 2.5 m ² / g or less)>
Lithium carbonate, nickel carbonate, and manganese carbonate were mixed at a predetermined ratio by a ball mill to obtain a precursor. The obtained precursor was calcined in the atmosphere at 500 ° C. for 12 hours to obtain a lithium transition metal oxide. After pelletizing the obtained lithium transition metal oxide, it was baked at 1000 ° C. or higher for 12 hours in the air. The fired pellets were pulverized in an agate mortar.

<Production of second positive electrode active material (specific surface area of 3 m ² / g or more)>
Lithium carbonate, nickel carbonate, and manganese carbonate were mixed at a predetermined ratio by a ball mill to obtain a precursor. The obtained precursor was calcined in the atmosphere at 500 ° C. for 12 hours to obtain a lithium transition metal oxide. The obtained lithium transition metal oxide was pelletized and then calcined at 950 ° C. or lower for 12 hours in the air. The fired pellets were pulverized in an agate mortar.

In the present embodiment, the composition ratio of the first and second positive electrode active materials is the same, the first active material having a large particle size and the second active material having a small particle size are mixed, and each Example 1 ~ 6 / Positive electrode materials of Comparative Examples 1-5. Table 1 shows the composition of the produced positive electrode material, the particle diameter and specific surface area of each positive electrode active material.

<Production of prototype battery>
In each example and comparative example, a positive electrode was produced using the positive electrode material produced as described above, and 11 types of prototype batteries were produced.

A positive electrode slurry was prepared by uniformly mixing a positive electrode material, a conductive agent, and a binder. The positive electrode slurry was applied onto an aluminum current collector foil having a thickness of 20 μm, dried at 120 ° C., and pressed at 40 MPa to obtain an electrode plate. Thereafter, the electrode plate was punched into a disk shape having a diameter of 15 mm to produce a positive electrode.

The negative electrode was produced using metallic lithium. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate and dimethyl carbonate having a volume ratio of 1: 2 was used.

<Charge / discharge test>
In each example and comparative example, a charge / discharge test was performed on 11 types of prototype batteries produced as described above. The charging was constant current constant voltage charging (CC-CV mode), and the upper limit voltage was 4.6V. The discharge was constant current discharge (CC mode), and the lower limit voltage was 2.5V. The charge / discharge current was 0.05 C, and the charge cutoff current was 0.005 C. In each example and comparative example, after two cycles were performed under the above conditions, the discharge capacity at the second cycle was defined as the rated capacity.

<Volume energy density ratio (5C) measurement>
The volume energy density was measured when the battery was charged to 4.6V under the same conditions as the charge / discharge test and discharged to 2.5V with a current corresponding to 5C. Table 1 shows the volume energy density ratio obtained by dividing the volume energy density of each example and comparative example by the value of comparative example 1.

<Measurement of OCV ratio>
In each of the examples and comparative examples, the difference between the OCV in the charging process and the OCV in the discharging process was determined for the 11 types of prototype batteries manufactured as described above.

The test of charging 10% of the rated capacity with a current equivalent to 0.05 C and waiting for 5 hours was repeated 10 times until the rated capacity reached 100%. Thereafter, a test of discharging 10% of the rated capacity at a current equivalent to 0.05 C and 10% of the rated capacity and waiting for 5 hours was repeated 10 times until the fully discharged state.

At this time, the voltage after 5 hours was defined as OCV. In this test, the voltage after charging from full discharge to 50% of rated capacity (SOC 50%) and waiting for 5 hours is the OCV during charging, discharging from full charge to 50% of rated capacity and waiting for 5 hours The voltage after the discharge was defined as the OCV of the discharge process. In each example and comparative example, the difference between the OCV in the charging process and the OCV in the discharging process was calculated, and the value divided by the difference between the OCV in the charging process and the OCV in the discharging process in Comparative Example 1 was defined as the OCV ratio. The results are shown in Table 1.

Example 1 is an example in which the specific surface area was reduced with respect to Comparative Example 1 while maintaining the same composition. The energy density at 5C was improved, and the OCV ratio could be lowered. By raising the firing temperature and increasing the crystallinity, it is speculated that the specific surface area decreased, contributing to improvement of rate characteristics and reduction of OCV hysteresis.

Furthermore, in Examples 2 and 5 in which a second active material having the same composition and a large specific surface area and a small particle diameter was mixed, the energy density ratio increased with an increase in the amount of the second active material added. On the other hand, the OCV ratio also tended to increase. Therefore, the amount of the second active material having a small particle size is preferably 40% or less.

Example 3 and Comparative Example 2 are examples in which the amount of Li was decreased without changing the ratio of Ni and Mn to Example 2, and Examples 4 and Comparative Example 3 were examples in which Li was similarly increased. In either case of Comparative Example 2 where the Li composition ratio is as small as 1.09 and Comparative Example 3 where the Li composition ratio is as large as 1.20, the discharge capacity is lowered and the volume energy density at a high rate is lowered. In addition, the OCV ratio tends to increase and the hysteresis tends to increase. Therefore, the composition ratio of Li needs to be in the range of 1.09 to 1.20. The ratio of Li to the metal element M (Li / M) is preferably in the range of 1.2 to 1.4.

Example 6 and Comparative Examples 4 and 5 are examples in which the ratio of Ni contained in the metal element (Ni / Ni + Mn) was changed. The discharge capacity decreased in both the case where the amount of Ni was small and the case where the amount of Ni was large. The reaction in which Ni contributes to reduction can be followed even in high-rate discharge, but the reaction in which oxygen contributes to reduction cannot follow. Therefore, when the amount of Ni decreases, the energy density at a high rate decreases. On the other hand, if the amount of Ni is too large, the ratio of Li ₂ MO ₃ that contributes to higher energy density of the layered solid solution is relatively reduced, and a sufficient energy density cannot be obtained. Furthermore, the OCV ratio increased as the proportion of Mn increased. In order to maintain a high energy density and reduce the OCV ratio, the ratio of Ni in the entire transition metal element (Ni / transition metal element M) is preferably 0.35 or more.

As described above, in Examples 2 to 6, the energy density ratio was higher than that in Example 1. This is because the second positive electrode active material having a specific surface area of 3 m ² / g or more is included. The second layered solid solution compound having a large specific surface area can achieve a high volumetric energy density, for example, by obtaining a high capacity and by improving the electrode density by mixing two materials having different specific surface areas. On the other hand, Comparative Examples 2 to 5 were unable to achieve both a higher discharge capacity and a smaller OCV ratio than Comparative Example 1.

As described above, the composition formula _{Li x Ni a Mn b M c} O 2 (1.09 <x <1.20,0.21 <a <0.42,0.42 <b <0.63,0 ≦ c ≦ 0.02, x + a + b + c = 2.0, M is represented by V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc.) and has a specific surface area of 2.5 m The positive electrode active material of ² / g or less obtained a high energy density even at a high rate, and resulted in a small OCV hysteresis during the charging and discharging processes. Further, the composition formula Li _{x ′} Ni _{a ′} Mn _{b ′} M _{c ′} O ₂ (1.09 <x _′ <1.20, 0.21 <a _′ <0.42, 0.42 <b _′ <0. 63, 0 ≦ c _′ ≦ 0.02, x _′ + a _′ + b _′ + c _′ = 2.0, M is at least one of V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc. The volume energy density at the time of high rate can be further improved by mixing the second positive electrode active material represented by the above element) and having a specific surface area of 3 m ² / g or more.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Battery can 5 Packing 6 Negative electrode lead piece 7 Sealing lid 8 Positive electrode lead piece 9 Insulation board 10 Lithium ion secondary battery

Claims (6)

1. The composition formula _{Li x Ni a Mn b M c} O 2 (1.09 <x <1.20,0.21 <a <0.42,0.42 <b <0.63,0 ≦ c ≦ 0.02 X + a + b + c = 2.0, M is a positive electrode active material represented by V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg, Cu, etc.), and has a specific surface area of 2 A positive electrode active material for a lithium ion secondary battery, wherein the positive electrode active material is 0.5 m ² / g or less.
2. The positive electrode active material for a lithium ion secondary battery according to claim 1,
   A positive electrode active material, wherein 0.32 <a <0.42 and 0.40 <b <0.53.
3. A positive electrode material for a lithium ion secondary battery,
   Including a first positive electrode active material and a second positive electrode active material,
   The first positive electrode active material is the positive electrode active material described in claim 1,
   The second positive electrode active material has a composition formula Li _{x ′} Ni _{a ′} Mn _{b ′} Mc _′ O ₂ (1.09 <x _′ <1.20, 0.21 <a _′ <0.42, 0. 42 <b _′ <0.63, 0 ≦ c _′ ≦ 0.02, x _′ + a _′ + b _′ + c _′ = 2.0, M is V, Mo, W, Zr, Nb, Ti, Al, Fe, Mg And a specific surface area of 3 m ² / g or more.
4. The positive electrode material for a lithium ion secondary battery according to claim 3,
   Said 2nd positive electrode active material is contained in the ratio of 40 mass% or less with respect to the total amount of said 1st and 2nd positive electrode active material, The positive electrode material characterized by the above-mentioned.
5. The positive electrode material according to claim 4,
   The first and second positive electrode active materials have the same composition,
   Said 2nd positive electrode active material is contained in the ratio of 20 mass% or less with respect to the total amount of said 1st and 2nd positive electrode active material, The positive electrode material characterized by the above-mentioned.
6. A lithium ion secondary battery using the positive electrode material according to any one of claims 3 to 5.

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